Event detection devices, for example, intrusion monitoring devices, are well known in the art. Typically they are used to detect unauthorized entry or intrusion into a protected space.
Commercially available intrusion monitoring devices can be either of the passive or active variety. Passive intrusion monitoring devices can, for example, comprise a sensor which detects infra red radiation propagated by warm blooded animals. Typically such passive devices comprise a thermal detection device, consisting of one of more thermal detectors adapted to detect infra red radiation incident thereon, and an optical system for directing incident radiation from a plurality of angular fields of view towards the thermal detection device. Such optical systems may consist of lenses, particular Fresnel lenses and/or reflecting surfaces. Normally such devices are activated when a source of infra red radiation passes from one angular sector to the next. Typical prior art intrusion monitoring devices are illustrated in U.S. Pat. Nos. 3,703,718 and 3,958,118, and in UK patent number 1335410. The entire disclosures of all these patents are included herein by reference for all purposes.
Active intrusion monitoring devices are also known which comprise a transmitter and a receiver, the transmitter emitting radiation at a defined frequency and a receiver measuring the Doppler shift in any reflective radiation. Such active devices can, for example, operate at microwave frequencies, using a microwave radiation detection device to detect the reflective radiation.
Whatever detection device is used, it is necessary in each case to provide an electrical circuit to process the electrical output signal of the detection device and to compare that signal with a pre-set threshold signal.
The above devices can be used alone, or as a combined technology event detection device. Examples of such combined devices including specifically a combination of a photo electric sensor and a microwave sensor are shown in U.S. Pat. Nos. 3,725,888 and 4,401,976, the entire disclosures of which are incorporated herein by reference for all purposes.
In a typical combined technology event detection device, the outputs of two independent sensing means, responding to different physical stimuli, are supplied to an AND gate, and if both sensing means register an event within a specified period of time, then an alarm is triggered. In this manner the incidence of false alarms occurring when only a single sensor means is used can be greatly reduced.
A problem with both single and combined technology event detection devices is that if the detector is masked, for example, by placing a screen in front of the detector which will absorb the microwave signals emitted by the microwave device, or which will block infra red signals and prevent them from reaching the passive infra red sensor, the event detection device is rendered inoperable.
Attempts have been made to overcome this problem by providing the event detection device with a separate system comprising an infra red LED emitter and a detector which operate at a frequency range different from that of the passive infra red sensor. If an object is placed near the event detection device so as to mask the passive infra red sensor, the infra red LED/detector system will detect the presence of the object and cause an alarm to be triggered.
Such anti-masking system increase the expense of the device, and in some circumstances are ineffective, because it is still possible to mask all or part of the Fresnel lens associated with the passive infra red sensor without traversing the light beam from the infra red LED. Thus a skilful thief can mask the lens without activating the anti-masking system.
U.S. Pat. No. 4,833,450 discloses an event detection in which the alarm is sounded if a signal from a masking circuit exceeds a threshold level. The alarm continues to sound for a predetermined period. Once the predetermined period has lapsed and the correct of operation of the event detection device has been confirmed, the alarm is reset.